This invention relates to fluid-responsive apparatus.
The invention is more particularly concerned with mass flowmeters and densitometers.
Flowmeters responsive to the rate of flow of mass of fluid are useful in many applications where the density of the fluid might change, such as, upon a change in temperature of the fluid. Mass flowmeters are used, for example, in aircraft where it is the mass, rather than the volume, of fuel supplied to an engine that determines the thrust produced by the engine.
In one form of presently available mass flowmeter (such as described in U.S. Pat. No. 2,714,310), an impeller mounted in the fluid flow path is rotated by a motor so as to give the fluid a swirling motion. The momentum imparted to the fluid by the impeller is used to rotate a turbine, mounted downstream, against the action of a spring. The torque applied to the turbine is measured and gives an indication of the mass flow of fluid. Various modifications and refinements of this arrangement have been proposed but they all require the provision of a rotating member and the consequent problems associated with the use of rotary bearings, such as their expense, relatively short life, and difficulties of lubrication.
An alternative flowmeter (such as described in U.S. Pat. No. 4,252,028) employs the Coriolis force for the measurement of mass flow, including a `U` shape tube through which the fluid flows. Drive is applied to vibrate the tube and sensors are used to measure the resulting Coriolis force, from which the mass flow can be determined. While such flowmeters are satisfactory where they can be rigidly mounted in a fixed location, they are bulky and sensitive to external vibration, making them unsuitable for many applications, such as, in aircraft.